These teachings relate generally to smoothing phosphors for AC LED lighting, and more particularly to smoothing phosphors being aluminate-based phosphors co-doped with at least manganese, Mn2+, and silicon, Si4+, that substantially reduce flicker of an AC LED (or array of AC LEDs) by way of prolonging the light emission time of an AC LED (or array of AC LEDs) during a ½ cycle response to a phase change of the alternating current.
As diodes, LEDs are direct current components and thus, when subjected to a sinusoidally alternating current (AC), such as provided by typical AC mains, LEDs will only produce light emission during half of the cycle resulting in significant loss of electrical efficiency. In order to take advantage of the full cycle, and therefore increasing electrical efficiency, numerous methods have been employed. For example, LED circuits are designed as rectifiers that invert the phase of the negative half of the cycle, making the full cycle of AC available for use with the LED. Alternatively, circuits are constructed with multiple LEDs such that the full cycle can be used for illumination. In light of these numerous methods employed, such AC-LED chips (or simply “AC-LEDs”) are becoming more attractive for use in low-cost general illumination. However, the light produced by AC-LEDs exhibit an unacceptably high degree of flicker, caused by the rapid alteration in polarity at mains frequency that requires the current to become zero at the points of inflection of the cycle. Since this frequency is typically around 50-60 Hz, this flicker can be irritating, particularly in the case of indoor lighting applications. Shifting the phase of one or more AC LEDs in an array is one possible method in reducing the flicker, but these electronic modifications introduce reactive power consumption, reducing the component's power factor and increasing harmonic distortion in the circuit. Thus, there is a need for alternative methods in addressing the existence of flicker with AC LEDs.
Currently, photoluminescent materials are used in the construction of LEDs to modify the emission by way of energy conversion, so as to provide longer wavelengths then are naturally emitted by the semiconductor chip. For example, cerium doped yttrium aluminum garnet phosphors are used to produce cool white LEDs. Characteristics of color quality, such as correlated color temperature and color rendering index, can be controlled by varying the amount and type of phosphor used for the energy conversion. Unfortunately, most phosphors currently employed with LEDs have an emission decay that is much faster, by several orders of magnitude, than the frequency of the flicker, and therefore are not useful in the reduction of flicker that results in AC LEDs.
There is, therefore, a need for phosphors which are capable of providing a longer emission decay thereby extending the light emission time of an AC LED (or array of AC LEDs) during a ½ cycle response to a phase change of the AC. It is further desirable to provide such phosphors that not only modify the emission of an AC LED (or array of AC LEDs), but also substantially reduce the flicker produced.